Energia solare termodinamica - Euroimpresa Legnano · Paolo SilvaPaolo Silva Energia solare termodinamica Prof. Paolo Silva Politecnico di Milano – Dipartimento di Energia [email protected]

Paolo Silva Paolo Silva

Energia solare termodinamica Prof. Paolo Silva Politecnico di Milano – Dipartimento di Energia [email protected] Legnano, 12 Novembre 2013

Energia al Trasferimento tecnologICO

mailto:[email protected]�

mailto:[email protected]�

Paolo Silva

Presentation Outline 2

The Solar Resource Introduction on Concentrating solar power Concentrating solar power components

• Plant configuration, heat transfer fluids and storage • Linear Solar collectors

Parabolic Trough Collector Linear Fresnel Collector

• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems

The Solar Resource

Paolo Silva

3 Solar energy

Advantages:

Renewable

Low environmental impact

Modular

Disadvantages:

Costly

Low density

intermittent energy source

Paolo Silva

Paolo Silva

4

on the earth surface 1000 W/m2

Solar radiation wavelenght: from 0.3 to 2.5 µm, peak at 0.5 µm The solar radiation through the atmosphere is weakened due to

scattering and absorption (CO2, H2O, O3).

The solar radiation is the electromagnetic energy emitted by the sun

The power emitted by the sun is 3.8*1014 TW

The maximum power density on a yearly average: outside the atmosphere: 1367 W/m2 (solar constant) it represent the

irradiation on a surface perpendicular to the line connecting the the earth and the sun

The Solar Resource

Paolo Silva

5 Solar spectrum

Visible=48% of the total radiation, UV=6%, IR=46% Annual average radiation 1,367 W/m2 out of atmosphere (±3% during year)

Clear sky, G=1000 W/m2

Paolo Silva

6

The effect of attenuation on the solar radiation is: • Albedo: is the amount of radiation reflected back by the atmosphere • Diffuse radiation (Gd): is the amount of radiation that arrives from all the

directions due to the scattering • Direct radiation (Gb): is the amount of radiation that is not deviated nor

absorbed and keep the direction of the sun rays

The Solar Resource

Paolo Silva

7 Solar radiation

Radiation reaching earth surface: Gglobal = Gbeam + Gdiffuse

Paolo Silva

8 The Solar Resource

Paolo Silva

9

• TMY3 (typical metereological year) data, mainly for US sites (but not only): http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html • European Data-base of daylight and solar radiation: http://www.satel-light.com/indexgS.htm

Online DNI data

The Solar Resource

Paolo Silva

PHOTOVOLTAICS THERMAL COLLECTORS THERMODYNAMIC

SYSTEM

Concentrating Solar Power

(CSP)

Systems w/o concentration

ELECTRIC ENERGY THERMAL ENERGY

Systems w/o concentration

(PV)

Concentrating Photovoltaic

(CPV)

SOLAR ENERGY

Introduction on Concentrating Solar Power

Paolo Silva

Solar energy

PV versus CSP:

• EPC costs for large scale PV plants: 1600 €/kW • EPC costs for large scale CSP plants: 3500 €/kW • PV cumulative installed capacity: 70 GW (2011) • CSP cumulative installed capacity: <3 GW (2011)

However… • CSP costs should drastically reduce in the future due to economy-of-scale effects (the thermodynamic conversion uses well-known technologies!) • Efficiency and costs should benefit from an increase in the plant size • CSP EPBT is lower than PV (1 year vs. 4-5 years) • CSP has the key to obtain “dispatchability”: decoupling of electricity production from the availability of the source

Thermal storage tanks obtain this in a relatively cheap way

Paolo Silva






Paolo Silva

13

θz

• Zenith Angle (θz): angle between the zenith (an imaginary point directly "above" a particular location) and the sun direction

• Solar Altitude (α): complementar of the Zenith angle • Azimuth Angle (γ): angle between North (South) and the projection of the sun

direction on the horizon plane Zenith

The Solar Resource

Paolo Silva Andrea Giostri – Claudio Saccilotto

Effective solar radiation on a surface

Paolo Silva

α = elevation γ = azimuth


Effective solar radiation on a surface

Paolo Silva

Gglobal,T = Gglobal * cos θ

θ


Solar thermal collectors are non appropriate for thermodynamic conversion…

Paolo Silva


Solar thermal collectors are non appropriate for thermodynamic conversion…

Paolo Silva

Tglobal

envrL

GTTU,

)( −−=ταη

Paolo Silva

Solar plants classification

Systems without concentration:

• Solar Pond use of salts in solution (NaHCO3) to create a vertical salinity gradient generating hot layers at depth (~ 80-90 ° C) and cold surface layers. In fact, the layers of salt solutions increase in concentration (and therefore density) with depth exploitation of the heat contained in the hot layers in a thermodynamic cycle (ORC "Organic Rankine Cycle") conversion efficiency solar-to-electricity is very low (~ 1 ÷ 2%)

• Plants based on “chimney effect“ – Updraft Tower consist of a greenhouse arranged around a tall tower with a wind turbine at the base the heat produced by solar radiation heats the air trapped in the greenhouse by means of natural convection air rises up the updraft tower. Large size of towers and huge extension of land is needed

Paolo Silva

Paolo Silva

Solar Pond at El Paso, Texas (USA)

Paolo Silva


Paolo Silva

Updraft Tower plant at Manzanares, Spain 50 kW 195 m height, 10 m diameter 46’000 m2

Paolo Silva



Why concentration?

Flabeg Thick Glass mirror

Paolo Silva


Absorber tube

Absorber tube with selective coating

Paolo Silva


Concentrating solar collector

,optical peak clη ρ γ τ α= ⋅ ⋅ ⋅

Riflessività Specchio (ρ) Assorbanza

Ricevitore (α)

Trasmittanza Intercapedine (τ) Radiazione

Solare Diretta

Fattore di Intercettazione (γ)

Paolo Silva

Paolo Silva

Parabolic Trough: Incidence angle

Andrea Giostri – Claudio Saccilotto Paolo Silva


Type of losses/1 Optical losses indipendent of the operating conditions depend only on the technology peak optical efficiency is defined (mirror and tube clean, θ = 0)

,optical peak clη ρ γ τ α= ⋅ ⋅ ⋅ Where: ρcl is the reflectivity of the clean mirror γ is the intercept factor τ is the glass trasmissvity α is the selctive coating absorbivity

Paolo Silva



Type of losses/2 Thermal losses related to the heat exchange between the absorber tube and the external environment related to the thermal properties of the absorber tube (emissivity) related to the thermodynamic properties of the fluid

Paolo Silva

R

CTbeam

envrL

AAG

TTU

,

)( −−= ργταη




Paolo Silva

DNI = 800 W/m2

Paolo Silva

Punctual: conc. in a singol point, high concentration factors

Continuous Partitioned

Solar Concentration

Goal

Increase heat fluxes Higher temperatures Lower receiver’s size Lower heat losses

Concentration types: Linear : concentrates solar beams over a line Continuous Partitioned

Parabolic trough Fresnel reflector

Solar Dishes (small size) Central receiver

(Solar Tower)

Disadvantage: concentrates only direct solar radiation

Paolo Silva

Paolo Silva

CSP Technologies

Line focusing sys: Parabolic Trough Line focusing sys: Fresnel

Point focusing sys: Solar Tower

Point focusing sys: Parabolic Dish

Paolo Silva

Concentrating Solar Plants classification

Systems with concentration:

• Concentration types: • Puntcual (Solar Tower, Solar Dish) • Linear (Parabolic Trough, Fresnel)

• Degrees of Freedom (DOF) of solar collectors (SC)

•1 DOF tipically used with linear concentration collectors (“dense array”) tracking system based on a single axis (E-W if orientation of SC is N-S) simple and cheap, but less efficient than 2 DOF (due to incidence angle θ)

•2 DOF tipically used with punctual concentration collectors (“point focus”) double axis tracking system higher efficiency, but good precision is required, higher costs

Paolo Silva

Paolo Silva







Solar Field Heat Storage system

Power Block

Paolo Silva

Introduction on Concentrating Solar Power

Paolo Silva

Main components of a CSP plant using linear collectors • Power Block water Rankine cycle for electric power generation

• Heat Storage aims to increase the yearly operating hours of the plant. It decauples supply and demand

• Connecting pipes are the link between the solar field and the power block • Solar Field consists of several files ("loops") of linear collectors (PT o LFR) responsible for uptake of solar radiation and transfer heat to the fluid flowing inside the absorbers tubes (HTF)

Linear concentration systems: Components

33

Paolo Silva

The Collector

34

Goal Increase heat fluxes Higher temperatures Lower receiver’s size Lower heat losses

Disadvantage: concentrates only direct solar radiation

Solar collectors capture incident solar radiation energy and convert it to heat (thermal energy)


Plant configurations • Indirect cycle the HTF heats the working fluid of the thermodynamic cycle

• synthetic oil (ongoing technology SEGS I – IX, Kramer Junction (US)) • molten salts (experimental technology Archimede Project, Priolo Gargallo, Siracusa)

• Direct Steam Cycle direct steam generation inside the receiver tubes of the collectors DSG technology (“Direct Steam Generation”) DISS Project, Almeria (Spain)

Paolo Silva

Plant configurations of a solar plant


Power Block

Purpose: to convert the thermal energy brought by the fluid into mechanical energy with an appropriate thermodynamic cycle and, consequently, into electrical energy through the alternator.

Type of thermodynamic cycle The main thermodynamic cycle used in commercial plants is the Rankine steam cycle (given the limited steam temperatures of 380 ~ 370°C). The possible variations are: presence of reheat (RH) arrangement of RH number of high-temperature regenerators type of condensation In point focus systems the thermodynamic cycle can be also a Joule or Stirling cycle.

Paolo Silva


Power Block

Paolo Silva

Solar field Solar field

Indirect cycle Direct cycle

Heat Transfer Fluid (HTF)


Power Block (Rankine cycle)

ECO

EVA

SH RH HP LPSOLAR FIELD

HT REGENERATORS LT REGENERATORS

Paolo Silva

Paolo Silva

SOLA

R F

IELD

SO

LAR

FIE

LD

1

2

3

4

5

6

7

8

Power plant layout: Direct Steam Generation (DSG)

Paolo Silva

Power plant layout: Direct Steam Generation (DSG) – Saturated steam

40


HTF

Types of heat transfer fluid/1 • Synthetic oil: it is the fluid used in all commercial systems (es. Therminol VP-1 in SEGS VI, Dowtherm A in Andasol I)

• ADVANTAGES:

Low freezing temperatures (~ 12÷20°C) High thermal stability in the range of operating temperatures Low viscosity (enhances heat transfer, minimizes problems of start-ups and pumping) No corrosivity (does not require the use of stainless steel or special alloys)

Paolo Silva


HTF

Types of heat transfer fluid/2

• DISADVANTAGES:

Limited maximum temperature (~ 400°C problems of thermal decomposition) Flash problems Problems of toxicity Working under pressure (~25÷35 bar) to prevent evaporation at the operating temperatures High costs (~4÷7 €/kg)

Searching for alternative heat transfer fluids

Paolo Silva


HTF

Types of heat transfer fluid/3 • Molten Salts: used in Archimede plant as HTF in the solar field or in Andasol I as storage fluid. There are various types of mixtures of molten salts: binary mixtures

• Solar Salts (%m/m: 60% NaNO3, 40% KNO3)

ternary mixtures • Hitec (%m/m: 7% NaNO3, 53% KNO3, 40% NaNO2) • Hitec XL (%m/m: 7% NaNO3, 45% KNO3, 48% Ca(NO3)2)

Paolo Silva


HTF

SOLAR SALT HITEC HITEC XL

T MAX [°C] 600 535 500

FREEZING POINT [°C] 238 142 120

DENSITY @ 300°C [kg/m3] 1899 1640 1992

VISCOSITY @ 300°C [cp] 3,26 3,16 6,37

HEAT CAPACITY @ 300°C [kJ/kg]

1495 1560 1447

Paolo Silva


HTF

Types of heat transfer fluid/4

• ADVANTAGES:

High maximum temperature (up to 600°C) No Flash problems No toxicity problems Low working pressure (~1÷10 bar) Low costs (~0,5÷2 €/kg)

• DISADVANTAGES:

High freezing temperature (difficulty in managing the night, eg. electric heating of tubes, or circulation of hot fluid)

Paolo Silva


HTF

Types of heat transfer fluid/5 • Steam/Water: the case of the DSG technology in which heat transfer fluid and the working fluid in the power cycle coincide

• ADVANTAGES:

No freezing problems No corrosivity or flash problems Virtually no cost

• DISADVANTAGES:

High pressures inside the absorber tube (~100 bar) Difficult to control the temperature along the tube

Paolo Silva

Paolo Silva

Storage system



Storage system

• Purpose: to ensure the operation of the system even during periods with no sunshine or during transients due to passing clouds mitigates the disadvantage of uncertainty / discontinuity of solar energy by increasing the operating hours of the plant

• State of the art: it is the component on which R&D efforts of the manufacturers are more focused. Still an open discussion about:

Optimal size of the storage Storage system technology Heat transfer fluid

Paolo Silva

Dispatchability is the real extra-value of CSP compared to other renewable energies


Storage system

Paolo Silva

0

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0 3 6 9 12 15 18 21 24

Po

we

r [k

W]

Hours

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0 3 6 9 12 15 18 21 24

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we

r [k

W]

Hours

Mid-season dayTypical summer day

Storage 1,5 hours Storage 7,7 hours Net power output Solar energy


Storage system

Optimal size of the storage related to the invetsment costs of the storage system depends on the plant operating strategy should minimize the “Levelized Electricity Cost” (LEC)

Storage system technology

• Type of heat transfer

Direct system (storage fluid = HTF in the solar field/receiver): does not require an intermediate heat exchanger Indirect system (storage fluid ≠ HTF in the solar field/receiver): requires an intermediate heat exchanger

Paolo Silva


Storage system

• Type of tank/1 Thermocline tank: one tank containing a fluid with a vertical temperature gradient, where the hot fluid is at the top while the cold fluid lies on the base. It can feature a low-cost filler material that performs three basic functions: it is the "bulk" of the heat capacity of the storage system it prevents convective mixing between high and low temperature zones reduces the amount of fluid required

Paolo Silva


Storage system

• Type of tank/2 Two tanks system: involves the use of two separated "tank", a hot tank and cold tank. The temperature levels depend on the maximum and minimum temperatures reached by the HTF in the solar field. Commercial plants adopting this technology are Archimede at Priolo Gargallo (direct system) and Andasol I near Sevilla (indirect system). In both cases the storage fluid is a molten salts mixture: Archimede (Thot,tank=550°C, Tcold,tank=290°C) Andasol I (Thot,tank=386°C, Tcold,tank=292°C)

Paolo Silva


Direct storage system (2 tanks)

Paolo Silva


Indirect storage system (2 tanks)

Paolo Silva


Indirect storage system (thermocline)

Paolo Silva


Storage system

Paolo Silva

Paolo Silva

Storage system

Storage system in ANDASOL I plant 2 indirect tanks (molten salts)


Paolo Silva






Paolo Silva

59

Parabolic Trough

The Parabolic Trough Collector


Parabolic Trough

SUPER IMMAGINE COPERTINA PARABOLIC TROUGH

Paolo Silva

Paolo Silva

Working principle of parabolic trough collectors • Solar radiation is concentrated by means of a parabolic mirror on a receiver tube located on the focus of the parabola • Inside the receiver tube (“absorber”) a HTF flows, which is progressively heated up. The HTF coming from the solar collectors is used as a heat source for the thermodynamic cycle

• The mirror and the absorber tube rotate togheter to track the sun

Mirror Absorber Tube

Tracking system

Support beams

61 The Parabolic Trough Collector

Paolo Silva

Supporting structure: it is a sort of collector frame and must meet three basic requirements:

1. Support the reflective mirrors and the absorber tube ensuring

optical alignment 2. Tolerate external forces (e.g. wind action) 3. Allow the rotation of the collector to track the Sun in its daily

apparent orbit

62

There are several commercial technologies (Luz LS2, LS3 Luz, EuroTrough, Solargenix, SKyTrough, HelioTrough…) that differ in module length, aperture of the collector and the material used to support beams.

The Parabolic Trough Collector: supporting structure

Paolo Silva

EuroTrough ET-100 collector w=5.76m, L= 100/150m

Solargenix SGX-2 collector w=5.76m, L=65m

63

SenerTrough collector w=5.76m, L=144m

Consorzio XXI collector (ENEA) w=5.90m, L=100m

The Parabolic Trough Collector: Supporting Structure

Paolo Silva

64

The research aims to: 1. Increase aperture 2. Create simplified structure 3. Create stiffer steel structure 4. reduce invest costs 5. increase performance

The Parabolic Trough Collector: supporting structure

Paolo Silva

Reflecting mirror: should concentrate the greatest possible amount of solar radiation on the absorber tube

high reflectivity (90÷96%) lightweight (easier to handle) adequate stiffness (minimizes the deformation of mirror under the action of dynamic loads) durability low specularity errors

65

Microscopic surface roughness cause scattering of the reflected ray with respect to the specular direction

The Parabolic Trough Collector: mirror

Paolo Silva

Flabeg Thick Glass mirror

66

THICK GLASS MIRROR (3-4 mm thick) [Flabeg et al.] Reflectivity ≈ 93-96% Cost ≈ 43-65 $/m2

THE STANDARD...


Paolo Silva

67

THIN GLASS MIRROR (1 mm thick) [Flabeg, ACG-Solar et al.] The Adhesive is a key point for durability Reflectivity ≈ 93-96% Cost ≈ 16-43 $/m2

ALUMINIZED MIRROR [Alanod et al.] Durability? (delamination) Reflectivity ≈ 90% Cost ≈ 22 $/m2

SILVERED POLYMERS (film) [SkyFuel, 3M et al.] Durability? Reflectivity ≈ 93% Cost ≈ 16 $/m2

... AND THE OTHER OPTIONS

• Only Aging tests have been performed

• The newst product have only 3 years of field experience

• The substrate cost is not negligible


Paolo Silva

68

ReflecTech by SkyFuel: a sample of Film mirror


Paolo Silva

Absorber tube: it is positioned in the focus of the parabola, and must transfer the captured energy to the HTF / working fluid that flows inside In a cross-section (transversal) can be recognized:

metal tube (Grade B, AISI 321H) of 70/80/90 mm selective coating (Ni, Cr, Al, Ti) air gap with a high vacuum (10-4 Torr) limits the convective heat losses protects the coating from oxidation coaxial tube of borosilicate glass

69 The Parabolic Trough Collector: the Absorber Tube

Paolo Silva

Schott PTR 70

70

Short wavelength: high absorptivity (and thus high emissivity) Long wavelength: low absorptivity (and thus low emissivity)

The Parabolic Trough Collector: the Absorber Tube

Paolo Silva

The Absorber tube is designed to optimize its optical properties, in order to maximize the optical efficiency of the collector: transmissivity of the glass tube ( τ = 93÷97% ) absorptivity of the selective coating ( α = 92÷97% ) In order to estimate the heat losses of the absorber tube a parameter of fundamental importance is: emissivity of the selective coating ( ε = f(T) ) for all the technologies is increasing with temperature The main loss mechanism are the radiative losses


Paolo Silva

0.05

0.1

0.15

0.2

0.25

0.3

300 350 400 450 500 550

Emis

sivi

tà ε

Temperatura [°C]

LUZ BLACK-CHROME

LUZ CERMET

UVAC 2008

UVAC MEDIO

SCHOTT

ENEA 0.89

0.9

0.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

LUZ BLACK

CHROME

LUZ CERMET

SOLEL UVAC

SOLEL UVAC 2008

SCHOTT PTR 70

ENEA

A

T

α

τ

Transmissivity and absorbivity of the absorber tube


Paolo Silva

Incidence Angle Angle formed by the normal to the opening of the mirror and the rays coming from the sun

Its value depends on several factors:

• Location of site (Latitude, Longitude) • day of the year • Time of the day • Direction of the tracking

strong influence on the energy performance of the system

73 The Parabolic Trough Collector: Optical Efficiency

Paolo Silva

74

Longitudinal Plane

Parabolic Mirror Aperture Area

Incidence Angle θ

Transversal Plane

θ

INCIDENCE ANGLE

The Parabolic Trough Collector: Optical Efficiency

Paolo Silva

75

Nominal Optical losses indipendent of the operating conditions depend only on the technology peak optical efficiency is defined (mirror and tube clean, θ = 0)

Mirror Reflectivity

(ρ) Coating

Absorbivity (α)

Glass Transmissivity (τ)

Direct Solar Radiation

Intercept Factor (γ)

The Parabolic Trough Collector: Optical Efficiency

Paolo Silva

There is an ‘image spread ’ due to:

• SUN SHAPE The sun view from the earth appear as a disk of angular radius of ∆s=4.7 mrad Usually its shape is approximated as a gaussian

0

0.2

0.4

0.6

0.8

1

1.2

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30

rela

tive

Inte

nsity

Angle (mrad)

CSR10

Normal, s=2.8 mrad

The Parabolic Trough collector: Optical Efficiency Intercept Factor

Paolo Silva

• SUN SHAPE • MIRROR SPECULARITY Microscopic surface roughness cause scattering of the reflected ray with respect to the specular direction



Paolo Silva


• SUN SHAPE • MIRROR SPECULARITY • SLOPE (CONTOUR) ERRORS

Deviation of the surface from the best fit parabola: The waviness, with typical wavelengths on the order of centimeters to decimeters; represents medium scale errors.


Paolo Silva

• SUN SHAPE • MIRROR SPECULARITY • SLOPE (CONTOUR) ERRORS • SHAPE ERRORS • ALIGNMENT ERROR

Is the difference between the design shape and the average shape: represents large scale optical errors, caused by gravity, wind, materials or manufacturing errors.



Paolo Silva


• SUN SHAPE • MIRROR SPECULARITY • SLOPE (CONTOUR) ERRORS • SHAPE ERRORS • ALIGNMENT ERROR • TRACKING ERRORS


Paolo Silva

• SUN SHAPE • MIRROR SPECULARITY • SLOPE (CONTOUR) ERRORS • SHAPE ERRORS • ALIGNMENT ERROR • TRACKING ERRORS

Each error type can be characterized by its rms angular width. The rms width σoptical for the total optical error is obtained by adding the squares of the individual widths:

OPTICAL ERRORS

Sun Shapeeam spread

Parabolic Trough: Optical Efficiency Intercept Factor

Paolo Silva

The Parabolic Trough Collector: Optical Efficiency (off-design)

Optical off-design losses (non zero incidence angle): effect of the incidence angle (K(θ)) decrease in energy density of radiation (cosθ) change in optical properties of the absorber (τ(θ),α(θ)) shadow projection of the support

mutual shading of the mirrors (ηshading) collector end losses (ηend_loss)

82

Paolo Silva

0

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0 10 20 30 40 50 60 70 80

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Incidenza (θ) [°]

LS2

LS3

ET-100

K(θ) trends for different collector technologies

83

( ) ( ) 4 5 2cos 5.251 10 2.8596 10ET

K θ θ θ θ− −= − ⋅ − ⋅


Paolo Silva

Reciprocal shadowing of mirrors in different hours of the day

Importance of spacing between rows of collectors (usually distance = 2.5 times the aperture of the parabola)


84

( )( )

cosmin max 0; ;1

coseff spacing Z

shadowing

W LW W

θη

θ

= = ⋅


Paolo Silva

Collector end-losses

Influenced by: • Lenght of receiver tube • Incidence angle • Position of the receiver tube (focal distance)


85

( )1 tan pmend loss i

abs

DL

η θ− = −

“Account for the spilling of radiation over the end of a finite trough”

From: P.Bendt et al, Optical analysis and optimization of line focus solar collector

Spillage of rays due to end losses


Paolo Silva

Thermal analysis purpose Calculation of solar collector performance: evaluation of the thermal power transferred to the fluid (estimation of thermal losses) evaluation of the pressure drop along the absorber tube Study of the influence of some technological-environmental parameters: direct radiation wind speed presence of H2 in the cavity

86 The Parabolic Trough Collector: Thermal Efficiency

Paolo Silva

Thermal losses related to the heat exchange between the absorber tube and the external environment related to the thermal properties of the absorber tube (emissivity) related to the thermodynamic properties of the fluid Need to model heat transfer in a cross-section of the absorber tube (hypothesis of circumferentially uniform temperature) To this extent it is very useful to introduce an "electrical analogy“ considering an equivalent circuit diagram


Paolo Silva

T_Fluid T_Cint T_Cext

T_GintT_Gext

T_Amb

T_Sky

q_conv_HTF q_cond_abs

q_rad_glass

q_conv_glass

q_cond_glass

q_conv_sky

q_rad_sky

q_cond_bracket


Inputs of the 1D model: - Materials

characteristics - Solar radiation hitting

the receiver - HTF temperature - Ambient Temperature

Output: - Temperature in each knot of the circuit - Fluxes and losses

The Model can be solved in subsequent sections along the tube lenght to perform a 2D analysis

Paolo Silva

T_Fluid T_Cint T_Cext

T_GintT_Gext

T_Amb

T_Sky

q_conv_HTF q_cond_abs

q_rad_glass

q_conv_glass

q_cond_glass

q_conv_sky

q_rad_sky

q_cond_bracket


Inputs of the 1D model: - Materials

characteristics - Solar radiation hitting

the receiver - HTF temperature - Ambient Temperature

Output: - Temperature in each knot of the circuit - Fluxes and losses

The Model can be solved in subsequent sections along the tube lenght to perform a 2D analysis

0 100 200 300 400 500 600 700 8000

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450

Collector Length [m]

Tem

pera

ture

s [°C

]

T_HTFT_in_steelT_out_steelT_in_pirexT_out_pirexT_ambT_sky

Paolo Silva


An example of sensitivity analysis as a function of the HTF temperature: model VS real measured data


Parabolic Trough: on-design sizing

Example of sizing of an indirect cycle plant (synthetic oil Therminol VP1)

• Net electric power: 50 MWe

• Technology: Absorber tube Solel, Structure Eurotrough, Mirror Flabeg

• n° SF section: 4 (“H”)

• SF global thermal efficiency: 65,61%

• Electric efficiency of PB: 31,75%

• Net electric efficiency (solar-to-electricity): 20,83%

• Solar Field Area (mirrors): 299985 m2

• SF Size LxW: 1140 m X 649 m

Paolo Silva


Yearly simulation of the plant

N-S

E-O

• N-S higher annual electricity production

• E-O greater constancy of productivity

through the year

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GIA

[MW

h]

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N-S

MEDIA E-O

MEDIA N-S

Effect of axis orientation:

Paolo Silva


N-S axis orientation (site of Almeria (Spain) (36° 50′ 0″ N, 2° 27′ 0″ W))

TIPOLOGIA IMPIANTO

ENERGIA ELETTRICA [MWh]

ORE EQUIVALENTI [h]

CICLO INDIRETTO AD OLIO 94’045 1’881

CICLO INDIRETTO A SALI FUSI 81’442 1’629

CICLO IBRIDO DSG + OLIO 91’836 1’837

CICLO IBRIDO DSG + SALI 78’819 1’576

E-O axis orientation (site of Almeria) TIPOLOGIA IMPIANTO

ENERGIA ELETTRICA [MWh]

ORE EQUIVALENTI [h]

CICLO INDIRETTO AD OLIO 83’179 1’664

CICLO INDIRETTO A SALI FUSI 71’774 1’435

CICLO IBRIDO DSG + OLIO 82’224 1’644

CICLO IBRIDO DSG + SALI 70’988 1’420

for both cases electricity

production is higher with N-S

orientation

Paolo Silva



Effect of geographic coordinate: Sites located at different latitude

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[MW

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N-S

MEDIA E-O

MEDIA N-S

Dakar (Senegal) (14° 41′ 0″ N, 17° 27′ 0″ W) DNI = 1947 kWh/m2

Almeria (Spain) (36° 50′ 0″ N, 2° 27′ 0″ W) DNI = 1957 kWh/m2

• Influence of the angle of incidence • Meteorological differences

Paolo Silva


Paolo Silva

Cantiere

Collettori

Accumulo

Generatore vapore

Gruppo termoelettrico

Ripartizione dei costi per sottosistemi

Investment Cost parameters

€/m, €/m2

€/m2

€/kg, €/m2 (…)

Ricevitori

Pannelli riflettenti

Struttura

Inseguimento

Piping collettori

Controllo

Piping esterno

Fondazioni

Altre opere civili

Fluido termovettore

Solar Field cost=270 €/m2 Total Investment Cost

(EPC) = 3500 €/kWel

95 Analysis of system cost allocation for PT plants

Paolo Silva






Paolo Silva

The Linear Fresnel collector

97

Paolo Silva

98 98

• The concept: approximation of the parabola with a series of flat mirrors

• The origin: a technology with “Italian” Roots: First Patent on LFR by G.Francia in 1962

The Linear Fresnel collector

Paolo Silva

• The direct solar radiation is focused by a series of mirrors on a linear absorber tube at a certain height above the mirrors • Inside the absorber tube a heat transfer fluid circulates, which is gradually heated: it represents the heat source of a thermodynamic cycle similarly to what happens in a parabolic-trough system


99 The Linear Fresnel collector

Paolo Silva

The REFLECTORS

• LFR use flat (or elastically bent) mirrors (w=0.5-1m) • The mirrors are cheaper , lighter and easier to clean • The number of mirrors can variate between 10 and 50 (higher CR)

100 The Linear Fresnel collector: the Mirrors

Paolo Silva

The SUPPORTING STRUCTURE The supporting structure is lighter beacuse the mirror are lighter Less foundation (lower wind load) The absorber tube is still above the primary mirrors (5-10 m) The absorber tower is kept still with a series of beams and steel wires

101 The Linear Fresnel collector: Supporting Structure

Paolo Silva

Fresnel collector: heat collection element (absorber)


Glass tube

Absorber tube

Single absorber tube Secondary concentrator

Absorber tubeGlass plate

Secondary receiver

Cavity

Absorber tubes

Insulation Insulation

Paolo Silva

The ABSORBER The absorber tube can be either evacuated or non evacuated Can be a single tube or a boundle of smaller tubes Can mount a secondary reflector above it to enhance ray interception

103

SUPERNOVA 1 ENEA

Evacuated absorber are the same used for PT collectors, but usually have also the secondary reflector

The Linear Fresnel collector: the Absorber tube/tubes

Paolo Silva

The ABSORBER • The absorber tube can be either evacuated or non evacuated • Can be a single tube or a bundle of smaller tubes • Can mount a secondary reflector above it to enhance ray interception

(CPC, Circular, Two wings... )

104

Insulation Insulation Single absorber tube Bundle of absorber tubes

Secondary Mirror Secondary Mirror

NOVA 1 AUSRA- AREVA


Paolo Silva

The ABSORBER • The absorber tube can be either evacuated or non evacuated • Can be a single tube or a bundle of smaller tubes • Can mount a secondary reflector above it to enhance ray interception

(CPC, Circular, Two wings... )

105

Non evacuated absorber are sealed inside the secondary reflector cavity (filled with air) to reduce thermal losses (convection) They are usually used for low/medium temperature (300°C)


Paolo Silva

Nominal Optical efficiency • Takes into account:

• Reflectivity of the primary and secondary mirror • Transmissivity of the cover glass or envelope glass • Absorptivity of the absorber tube • Cosine effect (a) • Shading (b) • Blocking (c)

106 The Linear Fresnel collector: Optical Efficiency

Paolo Silva

0

0.2

0.4

0.6

0.8

1

0 15 30 45 60 75 90

IAM

Incidence Angle [°]

IAM(θi)IAM(θ⊥)

107

Off desing efficiency

The Linear Fresnel collector: Optical Efficiency (off-design)

Paolo Silva

108

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Effic

ienc

y

Transversal Incidence Angle (°)

Cosine losses Shading/blocking spillover from primary mirrors Spillover from secondary mirror Overall optical efficiency

Scomposition of the optical losses with the variation of the transversal inxidence angle

The Linear Fresnel collector: Optical Efficiency (off-design)

Paolo Silva

The Linear Fresnel Collector: Thermal efficiency

109

• The thermal efficiency is defined as the thermal efficiency for parabolic trough and the same parameter ifluence the thermal performance of a LFR • Linear Fresnel Reflector that use evacuated absorbers can be modeled in the

same way made for PT.

• Linear Fresnel with non evacuated absorber can be modeled with more complex electric circuits. Also convection within the cavity has to be considered

Paolo Silva

110

• The thermal efficiency is defined as the thermal efficiency for parabolic trough and the same parameter ifluence the thermal performance of a LFR • Linear Fresnel Reflector that use evacuated absorbers can be modeled in the

same way made for PT.

• Linear Fresnel with non evacuated absorber can be modeled with more complex electric circuits. Also convection within the cavity has to be considered

• Another option is the simulation of the secondary reflector and of the absorber tube using a CFD software

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2

7.2

7.25

7.3

7.35

7.4

7.45

7.5

7.55

y [m

]

20000

40000

60000

30

210

60

240

90

270

120

300

150

330

180 0

Heat flux [W/m2]

The Linear Fresnel Collector: Thermal efficiency

Paolo Silva

Heat transfer fluids for Linear Collectors

111

Moving parts critical for solidification of salts The receiver and the connecting pipe are still

Types of Heat Transfer Fluid (HTF) • Synthetic oil (Therminol VP1, Dowtherm A): Tmax ~ 400°C

Is the ‘state of art’ HTF • Molten Salts: there can be solidification problems

Solar Salts (%m/m: 60% NaNO3, 40% KNO3) Tmax ~ 600°C Hitec (%m/m: 7% NaNO3, 53% KNO3, 40% NaNO2) Tmax ~ 535°C

Paolo Silva

112


Is the ‘state of art’ HTF • Molten Salts:

Solar Salts (%m/m: 60% NaNO3, 40% KNO3) Tmax ~ 600°C Hitec (%m/m: 7% NaNO3, 53% KNO3, 40% NaNO2) Tmax ~ 535°C • Steam/Water (DSG): High temperature mean high pressure. More promising for the Fresnel technology than PT.


Paolo Silva


Is the ‘state of art’ HTF

113

• Molten Salts: Solar Salts (%m/m: 60% NaNO3, 40% KNO3) Tmax ~ 600°C Hitec (%m/m: 7% NaNO3, 53% KNO3, 40% NaNO2) Tmax ~ 535°C • Steam/Water (DSG): High temperature mean high pressure. More promising for the Fresnel technology than PT.

• CO2 or other gas: just experimental facilities and some research on it There can be problems with the temperature distribution on the absorber tube

°C

0

20

40

60 Power per degree PT [W/m/deg]

LS-2, CR=22.74

Power per degree LFR [W/m/deg]


Paolo Silva

- Cheaper - Less land occupation - Easier to clean - Less drag-wind effect - More feasible for DSG and use of Molten Salts - No ball joints - Less tracking power consumption

Possible disadvantages of LFR VS PT - Lower on-design efficiency - Penalties at partial load

114

Possible advantages of LFR VS PT

114 Linear Collectors Comparison

Paolo Silva

115

The Andasol plants (Parabolic Trough): http://www.youtube.com/watch?v=bxCUYPzHsug The Puerto Errado 1 plant (Linear Fresnel Collector) http://novatecsolar.com/44-1-NOVATEC-TV.html

Linear Collectors Comparison: videos

http://www.youtube.com/watch?v=bxCUYPzHsug�



http://novatecsolar.com/44-1-NOVATEC-TV.html�







Paolo Silva







Parabolic Trough: Solar Field

Paolo Silva

Paolo Silva

Purpose: to transport the fluid from the solar field to the power block with the minimum thermal dissipation (presence of insulation) and low pressure drops. It consists of three main elements:

Headers for distributing and collecting the fluid to the loops of the solar field “U” links inside the loops Connection from the headers to the steam generator of the power block

Used Materials Piping (carbon steel, P91, AISI 316L / 321H) Insulation (mineral wool low thermal conductivity)

118 Piping System and Solar Field Layout

Paolo Silva

Sizing criteria Input data

Solar field layout Mass flow rate of fluid maximum speed of the fluid Thermodynamic conditions at inlet/outlet (temperature, pressure) Specifications for pipes (ASME)

Output data

Pipe sizing (thickness, diameter) Thinckness of insulation pressure drops Heat losses to the environment


Paolo Silva

• Usally in solar fields using PT each loop has a ‘U’ shape so the outlet and inlet of the

collector are on the same side • Central feed configuration is usually used but also a inverse return configuration is

possible (even Dp in each loop, higher costs)

120

2 tubi 3 tubi

Layout of the solar field:

Third Tube

Central field Inverse return

Piping System and Solar Field Layout

Paolo Silva

“I” configuration the total flow rate is divided in 2 sections

SEZIONE DEL CAMPO SPECCHI

POWER BLOCK

COLLEGAMENTO CAMPO SPECCHI PBLATO CALDO

HEADER CALDO

FILA

HEADER FREDDO

LOOP

POMPA CIRCOLAZIONE

121



Paolo Silva


POWER BLOCK

L_specchi

COLLEGAMENTO CAMPO SPECCHI PBLATO CALDO

HEADER CALDO HEADER FREDDO

FILA

LOOP

SEZIONE DEL CAMPO SPECCHI

COLLEGAMENTO CAMPO SPECCHI PBLATO FREDDO

“H” configuration the total flow rate is divided in 4 sections


Paolo Silva

Andasol I (Spain) – 50 MW 123

Paolo Silva

Piping and layout of the solar field

• LFRs have greater focal lenght and thus greater end-losses. • Usually LFRs use long parallel rows (few plant built)


PB

Paolo Silva

• The diameters of the connecting pipe change after every loop (theoretically) • The optimum piping size is a compromise between investment costs (material)

and operating costs (pumping power) • Extra meters of piping are needed to compensate for thermal expansion and

to allow access to each loop

Piping characteristics:


Paolo Silva






Paolo Silva

Efficiency Parameters

The efficiency parameters typically used to define the energy performance of a concentrating solar thermal power plant are: Optical efficiency of the solar field: Thermal efficiency of the solar field: Piping Efficiency:

127

Paolo Silva

Net Power Block efficiency:

Solar-to-electric efficiency (“overall efficiency”)

128

, , , _ , __

el turb el pumps el aux cond el net PBnet PB

boiler boiler

E E E EE E

η− −

= =

, _ , __

, _

el net PB el aux SFaux SF

el net PB

E EE

η−

=

Efficiency of Solar Field auxiliaries


Paolo Silva

129

PNET

ESUN

Optical losses E@absorber

EHTF

Collector Thermal losses

E@BOILER

Piping Thermal losses

PGROSS Condenser

losses

∆H PUMP SF

PAUX SF

PAUX + other losses

129

∆H PUMP


Paolo Silva

130 Solar field control strategy (off-design)

Power increases with HTF max T

Power increases at constant HTF temp. with mass flow

Best control strategy: HTF outlet temperature is fixed at its nominal value (es. 390°C) and the HTF mass flow rate is varied at part load, until the minimum HTF mass flow is reached; for lower sun radiations HTF mass flow is kept constant reducing HTF temperature at the solar field outlet. (optimal value of minimum HTF mass flow is about 50% of nominal value)

Paolo Silva






Paolo Silva

132

FLABEG

SOLEL

ET - 100

OPTICAL EFFICIENCY: 75%

132 LFR vs PT: the chosen PT tehcnology

Paolo Silva

133

OPTICAL EFFICIENCY: 67%

NOVATEC SOLAR

133 LFR vs PT: the chosen LFR tehcnology

Paolo Silva

134

I

A

D

H

C

G

B

F

E

HTF: Therminol VP1

Tmax HTF =391°C p = 13 bar

Tin turbine=371°C p = 100/18.3 bar Tcond=41.5°C

p = 0.08 bar

Tcond= 235°C p = 100 bar

“H” configuration

134 LFR vs PT: overall plant layout – Indirect cycles

Paolo Silva

135

A

H

B

C

D

G

I

F

E

Tout SF=270°C p =55 bar

X = 0.8

Tcond=41.5°C p = 0.08 bar

T=196°C p = 66 bar

“I” configuration

135 LFR vs PT: overall plant layout - DSG

Paolo Silva

DSG 52.52 1.40 0.85 43 -

77.2 11.01 0.19 50

289,101 593,205

136

IND-PAR IND-FRE Gross power [MW] 54.87 53.45 Steam cycle aux cons. [MW] 1.72 1.68 Cooling tower aux cons. [MW] 0.65 0.63 Number of operating flow paths 72 75 HTF mass flow [kg/s] 616 601 Steam mass flow @ HP turbine [kg/s] 61.3 59.9 Pump head [bar] 23.34 10.86 Solar field aux cons. [MW] 2.51 1.14 NET POWER [MW] 50 50 Total SCA aperture area [m2] 235,899 268,596 Total required land area [m2] 683,902 594,465

ηoverall [%] 23.53 20.69 ηoverall_modified [%] 21.25 20.69

136

0 10 20 30 40 50 60 70 80 90 100

eta_overall modified (%)

eta_auxSF (%)

eta_netPB (%)

eta_piping (%)

eta_thermal (%)

eta_optical modified (%)

IND-PAR IND-FRE DSG

19.25 19.25

LFR vs PT: on design performance

Paolo Silva

137

0

0.2

0.4

0.6

0.8

1

0 15 30 45 60 75 90

IAM

Incidence Angle [°]

IAM(θi)IAM(θ⊥)

PARABOLIC TROUGH FRESNEL

137 LFR vs PT: solar field off design

Paolo Silva

138

IND-PAR IND-FRE DSG Available solar energy [MWh/y] 609464 696185 749336

Receiver solar energy [MWh/y] 321473 264173 289700 Net electric energy [MWh/y] 97818 74454 76136 ηoptical (%) 52.75 37.95 38.66 ηoptical_modified (%) 49.17 37.95 38.66 ηthermal (%) 92.73 85.82 92.08 ηpiping (%) 98.64 98.25 99.75 ηnet_PB (%) 34.45 34.00 28.77 ηaux_SF (%) 96.57 98.29 99.47 ηoverall (%) 16.05 10.69 10.16 ηoverall_modified (%) 14.96 10.69 10.16

138 LFR vs PT: yearly performance

Partial load penalization: PT: -32% LFR: -48% and -47%

Paolo Silva

Some of the of existing PT plants

IMPIANTO ANNO SITO W EL, NETTA [MW] FLUIDO S [m2]

SEGS I 1985 KRAMER JUNCTION (USA) 13,8 OLIO 82960

SEGS II 1986 KRAMER JUNCTION (USA) 30 OLIO 190338

SEGS III / IV 1987 KRAMER JUNCTION (USA) 30 OLIO 230300

SEGS V 1988 KRAMER JUNCTION (USA) 30 OLIO 250560

SEGS VI 1989 KRAMER JUNCTION (USA) 30 OLIO 188000

SEGS VII 1989 KRAMER JUNCTION (USA) 30 OLIO 194280

SEGS VIII 1990 KRAMER JUNCTION (USA) 80 OLIO 464340

SEGS IX 1991 KRAMER JUNCTION (USA) 80 OLIO 483960

DISS PROJECT 1996 ALMERIA (SPAGNA) 5 ACQUA -----

ARCHIMEDE 2007 PRIOLO GARGALLO (ITALIA) 5 + Accumulo SALI FUSI 30156

ANDASOL I 2008 ALDEIRE-LA CALAHORRA (SPAGNA) 50 + Accumulo OLIO 510120

NEVADA SOLAR ONE 2008 BOULDER CITY, NV (USA) 64 OLIO 357000

Plants under construction: 1562 MW (Spain and USA)

139

Paolo Silva

140

Find out about existing CSP plants on: http://www.nrel.gov/csp/solarpaces/by_technology.cfm

IMPIANTO ANNO SITE W EL, NETTA [MW] FLUIDO S [m2]

Puerto Errado 1 2009 Calasparra (Spain) 1.4 (gross) Saturated Steam (270°C) 25800

Puerto Errado 2 2012 Calasparra (Spain) 30 Saturated Steam (270°C) 302000

Kimberlina 2008 Bakersfield (CA, USA) 5 Steam 26000

Llo solar thermal porject 2012 Llo (France) 9 Steam 120000

Alba Nova 1 2012 Corsica (France) 12 Saturated Steam 140000

Fera 2010 Sicily (Italy 1 ? Saturated Steam ?

Some of the existing LFR plants

Paolo Silva

Plants under construction

25 75 110

1405

0

200

400

600

800

1.000

1.200

1.400

1.600

America del Sud USA Africa Europa

MW

Solar tower

Parabolic through

Europe and Spain in particular appear largely predominant…

141

Paolo Silva

The plants under planning

350 1050

5635

100

3116

1600

300

0

2.000

4.000

6.000

8.000

10.000

12.000

Austarlia Sud Africa Africa Europa USA

MW

Fresnel reflector

Parabolic dish

Solar tower

Parabolic through

... on the other hand considering the plants at the design stage (PPA: Power Purchase Agreement), the dominance of the U.S. is evident...

142

Paolo Silva






Paolo Silva

SOLAR TOWER Main Characteristics • 2 DOF point focus concentration • Suitable for high power output (> 10 MW) Typical components • Heliostat high reflectivity moving mirrors (ex. Sheliostat ~ 100 m2) that concentrate solar radiation over a heat exchanger positioned on the top of the tower, the “solar receiver” mirrors require supports and electromechanical devices, i.e. “trackers”, that are electronically activated

•Solar Receiver a heat exchanger positioned on the top of the tower in which the concentrated sunlight is converted in thermal energy that is transferred to the HTF finally the HTF gives the thermal input to the power cycle

Paolo Silva

Point focus systems: Solar Tower

http://en.wikipedia.org/wiki/File:Heliostat.jpg�

Paolo Silva

• Tower its function is of supporting the solar receiver on the top. It can be made of steel (h < 120 m) or concrete (h > 120 m) Different technologies: • indirect system with molten salts Solar Two, Barstow (US) • indirect system with air TSA-Phoebus Project • direct system with air Refos Project • direct system with saturated steam (DSG) PS10 Project, Siviglia (ES)

Paolo Silva


Paolo Silva

• The concentration ratio is usually in the range of 500-1000 suns • The collector field is based on a large number of heliostats with a tracking control system to continuously focus direct solar radiation onto the receiver aperture area • Heliostats can be flat or also a parabolic shape with small curvature

• The optical efficiency of the solar field is equal to the ratio of the net power intercepted by the receiver to the product of the direct insolation and the total mirror area • Optical losses include the cosine effect, mirrors properties as reflectivity, shadowing, blocking, atmospheric attenuation and receiver absorbivity

Paolo Silva


mirrbeam

recopt AG

Q.

=η

Paolo Silva

CONFIGURATION: • North fields (the heliostats are just on the north side of the receiver): commonly used for high latitudes or high incidence angles •Surround fields (the heliostats surround the receiver): typical of location close to the equator

• Higher optical efficiency than linear collectors. Ex. Spain: nom. eff. = 77%, average eff. = 64%

Paolo Silva



PS10 Concentrating Solar Tower, Sevilla (Spain) 10 MW electricity, DSG saturated steam 40 bar

Paolo Silva



Point focus systems: Solar Tower (PS10 – Spain)

Paolo Silva

Paolo Silva

Indirect system with molten salts (%m/m: 60% NaNO3 - 40% KNO3 )

Solar Tower indirect cycle with direct storage (2 tanks)

Paolo Silva


Water steam Molten Salt Receiver

Outlet Temperature [°C] 250/525 566

Incident Flux [kW/m2] 350 550

Peak flux [kW/m2] 700 800

Maximum pressure [bar] 100-135 -

Thermal efficiency [%] 80-93 85-90

Paolo Silva

152 Solar tower (indirect cycle w/o storage)

SOLAR TWO (California)

Paolo Silva

Indirect cycle with air as HTF

Paolo Silva


Paolo Silva

Plant with gas turbine

Paolo Silva

Direct cycle with air (pre-heating of air coming from the compressor of a gas turbine)



Paolo Silva

ISCC

Paolo Silva

ISCC (integrated solar combined cycle)

Paolo Silva

Beam-Down solar concentration


Other concepts

BOILER

Heliostats

Central reflecting system

Heliostats

Solar Radiation Solar Radiation

Paolo Silva


Solar tower data (source: NREL)

Paolo Silva


Solar tower data

Paolo Silva


Solar tower data

Paolo Silva


Solar tower data

Paolo Silva






Paolo Silva

Point focus systems: Solar Dish

SOLAR DISH Main Characteristics • 2 DOF point focus concentration • Low power output for single collector (below 50 kW) Typical components • Concentrator paraboloid shape mirror with high reflectivity (ex. D=10 m, Pel ~ 25 kW) • Receiver delivers energy reflected from concentrator to the working fluid of the engine (designed for minimizing heat losses due to convection and irradiance)

Paolo Silva

Paolo Silva

Engine 2 different engines are adopted in commercial models:

•Stirling cycle pressurized circuit with a gas as working fluid (N2, He, H2) (Pmax ~ 20 Mpa, Tmax ~ 700°C, H2 and He yields high efficiency heat transfer)

• Joule-Brayton cycle low pressure gas as a working fluid (low efficiency heat transfer)

Paolo Silva

Point focus systems: Solar Dish

Paolo Silva

Dish Stirling

Paolo Silva

Paolo Silva SBP German/Saudi 50 kW

Cummins Power CPG 9 kW

STM e SAIC 25 kW

Dish Stirling

Paolo Silva

Dish Stirling – world efficiency record (31.3%)

Paolo Silva

Paolo Silva

168 Thermodynamics – ideal Stirling cycle

E

C

E

C

E

CE

E TT

QQ

QQQ

QL

−=−=−

== 11η ∫ ∫⋅1

2lnVVRT=

VdVRT=dVp=|L=|Q CCTT

1

4

3

2

1 2

3 4

Paolo Silva

Paolo Silva

169

CAUSES of NON-IDEALITY:

Heat transfer losses: (i) non-isothermal transformations (machines are adiabatic in nature, difficult to give / remove heat while the piston moves), (ii) heat exchanger ∆T (not infinite surfaces)

Fluid-dynamic losses: (i) pressure drops (regenerator), (ii) losses due to dead

volumes (volumetric efficiency not unitary)

Losses due to kinematics: adopted kinematics doesn’t allow to fully perform isochores

Electrical and mechanical losses

Paolo Silva

Thermodynamics – real Stirling cycle

Paolo Silva

170 Working fluid

Working fluid requirements:

Good heat transfer coefficients Low viscosity Thermal stability Low cost

Light-molecule fluids as H2 or He meet these requirements, but they do have

problems to escape through seals. H2 is also highly flammable

Heavy-molecule fluids as air or N2 are less suitable (bring lower η), however, is easier to make seals (corrosion at high T due to O2 presence in air has to be taken into account)

Paolo Silva

Paolo Silva

171 Configurations

α β

Configuration α: separate hot and cold piston Advantages: simplicity of the kinematics, good volumetric efficiency Disadvantages: difficulty in obtaining good seals

Configuration β: two coaxial pistons Advantages: seal are more simple to obtain, seals are less subject to wear (the

hot piston , said "displacer" , is subject to a ∆P equal to the pressure drops in the regenerator, the power piston is cold)

Disadvantages : kinematics is more complicated, volumetric efficiency is lower (dead volumes)

Paolo Silva

Paolo Silva

172 Configuration α

The regenerator is made of a metallic mass traversed by the fluid in both directions (other than a surface heat exchanger!): pressure drops, low volumetric efficiency, and is also subject to clogging (thermal cracking of oil that deposites in channels)

Paolo Silva

173 Configuration α

Paolo Silva

Paolo Silva

174 Configuration β

Paolo Silva

Paolo Silva

175 Configuration β − heater

Sometimes heat pipes with sodium can be used: it has excellent heat transfer properties (evaporates at 800°C at about 5 bar) Disadvantages: It is highly flammable (risk in case of failure)

Paolo Silva

Paolo Silva

176 Configuration β − free piston

Documents

Energia solare termodinamica - Euroimpresa Legnano · Paolo SilvaPaolo Silva Energia solare termodinamica Prof. Paolo Silva Politecnico di Milano – Dipartimento di Energia [email protected]